As signal processing performed by a microphone device which obtains outputs by processing signals received from two or more microphone units, there is a directivity synthesis method of a sound-pressure gradient type, for example. While the directivity synthesis method has an advantage that directivity can be formed on a small scale, the method has a disadvantage that the sensitivity to sound pressure is reduced when the directivity synthesis is performed. This is to say, according to the directivity synthesis method, although the directivity can be formed, the sensitivity to sound pressure is reduced with respect to a noise level of vibration noise caused in the microphone units. With this being the situation, when the directivity synthesis method is employed, the problem associated with vibration noise relatively becomes serious.
Conventional measures against vibration noise of microphones include: 1) Floating; 2) Cancelling using a vibration sensor; and 3) Cancelling using signals of microphone units. In the following, an explanation is given as to 2) Cancelling using a vibration sensor, which is closely related to the present invention as a method to address the problem of vibration noise.
FIG. 10 is a diagram for explaining a conventional method for addressing vibration noise. A microphone device 800 shown in FIG. 10 includes a microphone unit 1, a microphone unit 2 whose sound hole is sealed, a housing 3 which holds the microphone unit 1 and the microphone unit 2, and a signal subtraction unit 4 which receives an output signal from the microphone unit 1 and an output signal from the microphone 2 and performs subtraction of the received signals.
Next, an explanation is given as to an operation relating to processing performed to address vibration noise by the microphone device 800 configured as described so far.
The microphone unit 1 is set mainly for picking up a target sound wave, and provides an output signal of the picked-up target sound wave. Practically speaking, however, a diaphragm of the microphone unit 1 is vibrated by vibration caused by a factor other than the target sound wave. The vibration noise caused by this vibration is superimposed on the signal of the target sound wave to be picked up, and then an output of this superimposed signal is provided by the microphone unit 1.
In order to cancel this vibration noise, the microphone unit 2 is set as shown in FIG. 10. The sound hole of the microphone unit 2 is sealed in order for the sensitivity to sound waves to be reduced sufficiently, so that the microphone unit 2 operates as a vibration sensor. The microphone unit 2 is fixed in the housing 3 where the microphone 1 is fixed as well. With this configuration, the vibration caused by a factor other than the target sound wave would occur to the microphone 1 and the microphone 2 in the same way as much as possible.
In this way, the microphone unit 2 picks up the vibration noise, which also occurs to the microphone unit 1 and is caused by vibration resulting from a factor other than the target sound wave.
Thus, a vibration noise component of the output signal from the microphone unit 2 is considered to be the same as that of the output signal from the microphone unit 1. Also, through the subtraction processing performed by the signal subtraction unit 4, the vibration component superimposed on the output signal of the microphone unit 1 can be cancelled.
Accordingly, from the signal subtraction unit 4, the microphone device 800 can obtain the output of the sound wave signal which the microphone device 800 wishes to pick up.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 56-25892